Tailor welded blank, manufacturing method thereof, and hot stamped component using tailor welded blank

ABSTRACT

Disclosed are a tailor welded blank and a manufacturing method thereof. The tailor welded blank is manufactured by connecting blank elements of different materials or thicknesses, thereby eliminating quality problems in a welded zone. A hot stamped component is manufactured by hot stamping the tailor welded blank formed by laser-welding blank elements made of coated steel plates having different strengths or thicknesses using a filler wire, wherein a laser-welded zone has a martensite structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No.10-2012-0139035 filed on 3 Dec. 2012, and all the benefits accruingtherefrom under 35 U.S.C. §119, the contents of which is incorporated byreference in its entirety.

BACKGROUND

1. Technical Field

The present invention relates to a tailor welded blank which ismanufactured by connecting blanks of different materials or thicknesses,a manufacturing method thereof, and a hot stamped part component usingthe tailor welded blank. More particularly, the present inventionrelates to a tailor welded blank capable of preventing occurrence ofquality problems in a welded zone, a manufacturing method thereof, and ahot stamped component using the tailor welded blank.

2. Description of the Related Art

Respective parts of a vehicle employ various components having differentstrengths according to functions thereof. For example, a vehicle partfor absorbing impact upon collision may employ components havingrelatively low strength, and a vehicle part required to maintain itsshape for securing a survival space for occupants may employ componentshaving relatively high strength.

When a shock-absorbing part of a vehicle has high strength, theshock-absorbing part transfers impact to other parts instead of suitablyabsorbing the impact upon collision, whereby occupants and other vehicleparts can be exposed to excessive impact.

To fulfill continuous demand for weight and cost reduction in the fieldof vehicles, a single component is required to have different strengthsaccording to sections thereof.

That is, some sections of a component are required to have relativelyhigh strength for protection of occupants, and other sections thereofare required to have relatively low strength for impact absorption.

One example of such a component includes a B-pillar of a vehicle.

FIG. 1 is an enlarged perspective view of a chassis and a B-pillar of avehicle.

In a certain car, a B-pillar 1 is disposed between a front door and arear door and connects a bottom surface of a chassis to a roof.

In the B-pillar 1, an upper portion la is required to have a highertensile strength than a lower portion lb thereof. The reason for theprovision of differences in strength according to sections of theB-pillar provided as a unitary component is because there are twosections in the unitary component, i.e. a structural section (forexample, an upper portion required to support a vehicle roof when thevehicle overturns) that needs to maintain its shape upon collision and ashock-absorbing section (for example, a lower portion having a highpossibility of lateral collision with other vehicles) that needs to becrushed to absorb shock.

That is, the upper portion 1 a of the B-pillar 1 needs high strengthbecause it needs to maintain its shape upon vehicle collision in orderto secure a safe space to prevent occupant injury, whereas the lowerportion 1 b of the B-pillar 1 needs relatively low strength because itneeds to be deformed to absorb impact upon vehicle collision. If thelower portion 1 b of the B-pillar 1 has high strength as in the upperportion 1 a, the lower portion 1 b of the B-pillar 1 cannot absorbimpact upon lateral collision, whereby impact can be transferred toother structural members.

Although detailed strength can vary according to types or shapes ofvehicles, the upper portion 1 a of the B-pillar 1 needs a tensilestrength of about 1500 MPa, and the lower portion 1 b of the B-pillar 1needs a tensile strength of about 500 to 1,000 MPa.

In the related art, a component is first made of a low strength materialand a separate reinforcing material is attached to a section required tohave high strength. Further, when a unitary component is required tohave different strengths according to sections, a material having highhardenability (or a thick material) and a low strength material havinglow hardenability (or a thin material) are laser-welded to form a blank,followed by forming a final product through hot stamping.

A blank manufactured by laser-welding components of different materials(or thicknesses) is referred to as a tailor welded blank, which may alsobe manufactured using a coated steel plate.

When the coated steel plate is laser-welded, a composition material of acoating layer is fused and enters a molten pool of a welded zone,whereby the welded zone has different physical properties than a basematerial thereof.

For example, when the coating layer is an Al—Si or Zn-based layer,coating components can enter the welded zone upon laser-welding, therebycausing deterioration in physical properties of the welded zone.

In the related art, laser welding is performed after removing a coatinglayer from a section to be subjected to laser welding, therebypreventing the coating components from intruding into the welded zone.However, this method causes increase in manufacturing costs due toaddition of a process of removing the coating layer.

One example of a background technique is disclosed in Korean PatentPublication No. 10-2009-0005004A (published on Jan. 12, 2009), entitled“Method for manufacturing a weld component with very high mechanicalcharacteristics from a coated lamination sheet”.

BRIEF SUMMARY

It is an aspect of the present invention to provide a tailor weldedblank having excellent quality in a welded zone without removing acoating layer from the welded zone, and a manufacturing method thereof.

It is another aspect of the present invention to provide a hot stampedcomponent using the tailor welded blank, in which the welded zone has amartensite structure after hot stamping.

In accordance with one aspect, the present invention provides a methodof manufacturing a tailor welded blank, which includes: laser-weldingblank elements made of coated steel plates having different strengths orthicknesses using a filler wire.

The filler wire may have a composition, in which a welded zone does notgenerate a ferrite structure at temperatures ranging from 800° C. to950° C., by taking intrusion of a composition of coating layers of thecoated steel plates into the welded zone into account.

The coated steel plate may include an Al—Si coating layer, and thefiller wire may have a higher amount of an austenite-stabilizing elementthan a base material of each of the coated steel plates. Here, theaustenite-stabilizing element may include C or Mn.

The C content of the filler wire may be 0.1% by weight (wt %) to 0.8 wt% higher than that of the base material of the coated steel plate, andthe Mn content of the filler wire may be 1.5 wt % to 7.0 wt % higherthan that of the base material of the coated steel plate.

In accordance with another aspect, the present invention provides atailor welded blank manufactured by laser-welding blank elements made ofcoated steel plates having different strengths or thicknesses, using afiller wire, wherein a laser-welded zone has a composition providing anaustenite structure at temperatures ranging from 800° C. to 950° C.

The welded zone may have a composition in which a base material and acoating layer of each of the coated steel plates are mixed with thefiller wire.

In accordance with a further aspect, the present invention provides ahot stamped component manufactured by hot stamping a tailor welded blankformed by laser-welding blank elements made of coated steel plateshaving different strengths or thicknesses using a filler wire, wherein alaser-welded zone has a martensite structure.

Each of the blank elements may include a boron steel plate havinghardenability as a base material and an Al—Si or Zn coating layer, andthe filler wire may have a higher amount of an austenite-stabilizingelement than the base material of the coated steel plate. Here, theaustenite-stabilizing element may include C or Mn.

According to the present invention, a tailor welded blank ismanufactured using a filler wire that is designed to prevent a fusedcoating layer from intruding into a welded zone of the blank, so thatthe welded zone has a full martensite structure after hot stamping.

Therefore, in manufacture of the tailor welded blank, removal andrecoating of the coating layer are not needed, thereby obtaining costreduction and improved productivity.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will become apparent from the detailed description of thefollowing embodiments in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic view of a B-pillar and a chassis structure of avehicle;

FIG. 2 shows a microstructure of a welded zone formed by laser weldingwithout removing an Al—Si coating layer and a graph depicting hardnessdistribution of the welded zone before and after hot stamping;

FIG. 3 is a diagram of a method of manufacturing a tailor welded blankaccording to one embodiment of the present invention; and

FIG. 4 is a graph depicting shift of a transformation curve according toincrease in an amount of an austenite-stabilizing element.

DETAILED DESCRIPTION

Embodiments of the invention will now be described in detail withreference to the accompanying drawings.

It should be understood that the present invention is not limited to thefollowing embodiments and may be embodied in different ways, and thatthe embodiments are given to provide complete disclosure of theinvention and to provide a thorough understanding of the invention tothose skilled in the art. The scope of the invention is defined only bythe claims. Like components will be denoted by like reference numeralsthroughout the specification.

First, a method of manufacturing a tailor welded blank will bedescribed.

The present invention relates to a method of manufacturing a tailorwelded blank by laser-welding blank elements of different materials orthicknesses, in which the tailor welded blank has different physicalproperties according to sections thereof. Specifically, the presentinvention is aimed at solving a problem that, when blank elements aremade of coated steel plates, coating layers of the coated steel platesare fused and intrude into a welded zone upon laser welding, therebycausing defects in the welded zone.

When the blank elements made of the coated steel plate are laser-weldedtogether, the coating layer of the coated steel plate is fused andintrudes into the welded zone.

That is, the welded zone has a composition in which a base material ismixed with the coating layer of the coated steel plate, whereby thewelded zone has different physical properties than the base material.

Particularly, when the tailor welded blank is subjected to hot stamping,intrusion of the coating layer components into the welded zone causesreduction in strength of the welded zone.

For hot stamping, the tailor welded blank is mainly composed of a basematerial formed of a boron steel plate having hardenability, and anAl—Si coating layer formed on the base material. Thus, the welded zonecontains the components of the welded coating layer and has acomposition in which the Al and Si content increases above that of thebase material. As a result, after hot stamping, the welded zone has astructure in which martensite and ferrite coexist, instead of the fullmartensite structure. As a result, the welded zone undergoes reductionin strength due to coexistence of the ferrite structure and themartensite structure.

The present invention is intended to design a composition of a fillerwire by taking into account intrusion of the fused coating layer intothe welded layer during laser welding of the blank elements made of thecoated steel plate, and to provide desirable properties to the weldedzone using the designed filler wire.

FIG. 2 shows a microstructure of a welded zone formed by laser weldingwithout removing an Al—Si coating layer, and graphs depicting hardnessdistribution of the welded zone before and after hot stamping.

As shown, before hot stamping (indicated by a blue graph), a welded zonehas higher hardness than the base material, and a thermally-affectedzone by welding has the highest hardness.

On the other hand, after hot stamping (indicated by a red graph), thehardness of the base material increases, whereas the hardness of thewelded zone decreases.

Consequently, after hot stamping, the hardness (300 to 350 Hv) of thewelded zone becomes lower than the hardness (500 Hv) of the basematerial.

Table 1 shows results of tensile testing in which a laser-welded samplehaving a coating layer is subjected to tensile force in a directionperpendicular to a welding line.

As can be seen from the result, the welded zone has both lower yieldstrength and tensile strength than the base material, and has anelongation not more than 10% of the elongation of the base material.

This results from the intrusion of the components (Al or Si) of thecoating layer into the welded zone, and thus it can be seen that thewelded zone decreases in both hardness and strength after hot stamping,providing a high possibility of failure at the welded zone upon vehiclecollision.

FIG. 3 is a diagram of a method of manufacturing a tailor welded blankaccording to one embodiment of the present invention.

As shown, the method includes welding blank elements 10, 20, whichinclude base materials 12, 22 and coating layers 14, 24 formed on thebase materials, by a laser beam 30 using a filler wire 40, such thatcomponents of the filler wire 40, the base materials 12, 22 and thecoating layers 14, 24 coexist in a welded zone.

According to the present invention, a composition of the filler wire 40that will be fused and intrude into the welded zone is controlled toregulate a composition of the welded zone, thereby allowing the weldedzone to have desired physical properties.

Each of the base materials 12, 22 may employ a boron steel plate havinghardenability, and each of the coating layers 14, 24 may employ an Al—Sior Zn coating layer.

Considering intrusion of the coating layers 14, 24 into the welded zone,it is noted that the filler wire 40 has a composition which does notallow a ferrite structure to be formed in the welded zone attemperatures ranging from 900° C. to 950° C.

That is, the filler wire 40 has a higher amount of anaustenite-stabilizing element than the composition of the coated steelplate base material such that the welded zone can have a full austenitestructure without forming the ferrite structure even upon intrusion ofthe fused coating layer into the welded zone.

FIG. 4 is a graph depicting shift of a transformation curve according toincrease in an amount of an austenite-stabilizing element.

As shown, as the amount of the austenite-stabilizing element (such as C,Mn, etc.) increases, a temperature point Ac3 decreases and thetransformation curve shifts to the right, thereby increasing anaustenite structure loop.

In the present invention, the austenite-stabilizing element may includeC or Mn.

When the filler wire includes a higher amount of theaustenite-stabilizing element than the base material, the welded zonehas a full austenite structure at hot stamping temperatures ranging from800° C. to 950° C., and upon quenching of the welded zone, the fullaustenite structure is transformed into the full martensite structure.

In other words, even when the components of the coating layer intrudeinto a fused pool of the welded zone, the welded zone has the fullmartensite structure after hot stamping, because the filler wirecontains a great amount of the austenite-stabilizing element.

Therefore, even though the coating layer is not removed and thecomponents of the coating layer intrude into the welded zone, the weldedzone is prevented from being decreased in hardness and strength, andthus has similar physical properties to the base material, therebypreventing failure of the welded zone upon collision.

In order to prevent generation of the ferrite structure in the weldedzone due to intrusion of Al in the coating layer, the Mn content of thefiller wire is 1.5 wt % to 7.0 wt % higher than that of the boron steelbase material.

If the Mn content is below this range (i.e. below 1.5 wt %), theaustenite loop is not so much enlarged, so that the ferrite structurecoexists with the austenite structure in the welded zone at temperaturesranging from 800° C. to 950° C. Further, if the Mn content is above thisrange (i.e. above 7.0 wt %), the fused pool has decreased viscosity anda coefficient of expansion increases upon liquid-solid transformation,causing problems in the welded zone, such as shape qualitydeterioration, cracking, and the like.

Further, the C content of the coating layer may be 0.1 wt % to 0.8 wt %higher than that of the boron steel base material.

If the C content is lower than this range, the austenite loop is not somuch enlarged, so that the ferrite structure coexists with the austenitestructure in the welded zone at temperatures ranging from 800° C. to950° C. Further, if the C content of the coating layer is above thisrange, the welded zone excessively increases in hardness and strength,causing problems in the welded zone such as failure upon collision.

As a result of evaluation, when SABC1470 steel plates each comprising:0.22 wt % of C; 0.24 wt % of Si; 1.19 wt % of Mn, 0.0015 wt % of P;0.0006 wt % of S; and the balance of Fe and unavoidable impurities werelaser-welded without using a filler wire, a welded zone underwentfailure upon tensile testing. Further, when the steel plates werelaser-welded using a filler wire containing 0.6 wt % of C, the basematerial of the steel plate underwent failure.

As such, according to the present invention, blank elements made ofcoated steel plates having different strengths and thicknesses arelaser-welded using a filler wire to provide a tailor welded blank, inwhich a laser-welded zone has a composition providing an austenitestructure at temperatures ranging from 800° C. to 950° C. Therefore, thewelded zone of the tailor welded blank does not suffer from reduction instrength and hardness even after hot stamping.

Although some exemplary embodiments have been described herein, itshould be understood by those skilled in the art that these embodimentsare given by way of illustration only, and that various modifications,variations and alterations can be made without departing from the spiritand scope of the present invention. Therefore, the scope of the presentinvention should be limited only by the accompanying claims andequivalents thereof.

What is claimed is:
 1. A method of manufacturing a tailor welded blank,comprising: laser-welding blank elements made of coated steel plateshaving different strengths or thicknesses using a filler wire.
 2. Themethod according to claim 1, wherein the filler wire has a compositionsystem in which a welded zone does not generate a ferrite structure attemperatures ranging from 800° C. to 950° C., by taking intrusion of acomposition of coating layers of the coated steel plates into the weldedzone into account.
 3. The method according to claim 1, wherein each ofthe coated steel plates comprises an Al—Si coating layer, and the fillerwire has a higher amount of an austenite-stabilizing element than acomposition of a base material of each of the coated steel plates. 4.The method according to claim 3, wherein the austenite-stabilizingelement comprises C or Mn.
 5. The method according to claim 4, whereinthe C content of the filler wire is 0.1 wt % to 0.8 wt % higher thanthat of the base material of the coated steel plate.
 6. The methodaccording to claim 4, wherein the Mn content of the filler wire is 1.5wt % to 7.0 wt % higher than that of the base material of the coatedsteel plate.
 7. A tailor welded blank manufactured by laser-weldingblank elements made of coated steel plates having different strengths orthicknesses using a filler wire, wherein a laser-welded zone has acomposition providing an austenite structure at temperatures rangingfrom 800° C. to 950° C.
 8. The tailor welded blank according to claim 7,wherein the welded zone has a composition in which a base material and acoating layer of each of the coated steel plates are mixed with thefiller wire.
 9. A hot stamped component manufactured by hot stamping atailor welded blank formed by laser-welding blank elements made ofcoated steel plates having different strengths or thicknesses using afiller wire, wherein a laser-welded zone has a martensite structure. 10.The hot stamped component according to claim 9, wherein each of theblank elements includes a boron steel plate having hardenability as abase material, and an Al—Si or Zn coating layer.
 11. The hot stampedcomponent according to claim 10, wherein the filler wire has a higheramount of an austenite-stabilizing element than the base material of thecoated steel plate.
 12. The hot stamped component according to claim 11,wherein the austenite-stabilizing element comprises C or Mn.